Morphological patterns of sexual selection in the diving beetle Graphoderus liberus

Hypotheses: Individuals in good condition are better able to invest in costly traits associated with sexual selection and sexual conflict than individuals in poor condition. Sexual selection favours males with male genitalia that are appropriate for stimulating any encountered female (‘one size fits all’ hypothesis) and thus the allometric slopes of genitalia should be equal to 0. Organism: The diving beetle Graphoderus liberus. Time and place: July 2004; Swan Lake near Sudbury, Ontario, Canada. Analytical methods: Multivariate analysis to identify dimorphic traits. Analysis of body condition (via size-corrected mass) and correlations with dimorphic traits and genitalia. Allometric analyses of size of male genitalia. Traits studied: Protarsal pads and pronotum (male traits used for grasping females when copulating). Results: Females tended to be larger than males but males had larger pronota. Both the area of the male protarsal pad and pronotum length were proportional to the condition of the individual. Female pronotum length did not depend on the condition of the individual. The length and width of male genitalia did not depend on condition, and their allometric slopes did not differ significantly from zero. Conclusion: Traits associated with sexual selection and sexual conflict conformed to sexual selection theory. Males in better condition have larger structures for grasping females when copulating. In contrast, male genitalia conform to the ‘one size fits all’ hypothesis.